The invention relates to an apparatus for the recombination of hydrogen in a gas mixture, in particular for a nuclear power station.
In a nuclear power station, if there are situations in which an incident or accident occurs, however unlikely these may be, oxidation of zirconium could occur, for example, due to the heating of the core. In this case, the formation and release of hydrogen gas and carbon monoxide within the safety vessel or containment enclosing the reactor core must be expected. Explosive gas mixtures may consequently be generated within the containment.
Various devices or methods are under discussion for preventing the formation of explosive gas mixtures of this kind in the containment of a nuclear power station. These include, for example, devices, such as catalytic recombinators, catalytically and electrically operated ignition devices or the combination of the two devices mentioned above as well as methods for permanently or subsequently inertizing the containment.
When a catalytic recombinator is used for eliminating hydrogen from the atmosphere of the containment, early flameless recombination of hydrogen with oxygen is to be achieved in particular. In this case, a significant pressure build-up as a consequence of virulent hydrogen combustion is to be reliably avoided. An early-starting recombination device suitable for this purpose, which, even when dwelling for a relatively long time in the containment atmosphere, does not lose activity to any appreciable extent and starts passively at low ambient temperatures, is proposed in earlier, commonly assigned German Patent Application 196 36 557.0. By use of a recombination device of this type, xe2x80x9csmoothxe2x80x9d recombination of hydrogen in a, for example, steam-inertized phase of the containment atmosphere, without ignition, is possible. A catalyst system for the recombination of hydrogen with oxygen is also known from U.S. Pat. No. 5,167,908.
Published, European Patent Application EP 0 436 942 A1 discloses a recombinator system with a housing protection device which opens automatically as a function of an external temperature.
By contrast, in a state of readiness of the recombination system, the housing protection device is closed, so as to avoid the catalytically active surface of the recombinator being contaminated.
By contrast, in a recombinator device known from Published, European Patent Application EP 0 416 140 A1, filter media are provided, which retain the pollutants from the ambient atmosphere, such as, for example, aerosols, and thus protect the catalyst of the recombinator device against contamination.
Published, European Patent Application EP 0 388 955 A1 discloses a recombinator device, in which an ignition device is additionally provided for triggering controlled hydrogen combustion.
Every known recombinator system is configured for a particularly high recombinator capacity, along with particularly small component dimensions, and for high resistance to contamination. However, to use an apparatus for the recombination of hydrogen in a gas mixture in a nuclear power station, it is also necessary, furthermore, to ensure that no effects adversely influencing the safety of the nuclear power station can occur. It must be remembered that a catalyst configuration used for the recombination of hydrogen normally heats up as a result of recombination and, on account of its increased temperature, could contribute unintentionally to ignition of the gas mixture within the containment atmosphere of the nuclear power station.
It is accordingly an object of the invention to provide an apparatus for the recombination of hydrogen in a gas mixture that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which apparatus unintentional ignition of the gas mixture is avoided in a particularly reliable way.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for a recombination of hydrogen in a gas mixture, including a housing; a catalyst configuration disposed in the housing and through which a gas mixture can flow by free convection in an operating situation; and a flame retention device associated with the catalyst configuration and having a number of inflow orifices formed therein for an inflow of the gas mixture and the inflow orifices having an average size of at least 0.2 mm and of at most 3 mm.
For an apparatus of the above-mentioned type, the object is achieved, according to the invention, in that the associated catalyst configuration, which is disposed in the housing through which the gas mixture can flow by free convection in the operating situation, is assigned the flame retention device. The flame retention device has a number of inflow orifices provided for the inflow of the gas mixture and have an average size of more than 0.2 mm and at most 3 mm.
The terminology xe2x80x9cfree convectionxe2x80x9d, in particular, means that a local increase in temperature of the gas mixture, the increase resulting from the recombination of hydrogen in the vicinity of the catalyst configuration, results in a lift which overcompensates the pressure loss in the recombination apparatus in such a way, that the flow of the gas mixture through the recombination apparatus is ensured without external drive devices. In this case, the housing is configured in the manner of an approximately vertically disposed well, so that, in the operating situation, a chimney effect occurs so as to assist the flow of the gas mixture.
The dimensioning of the inflow orifices ensures the functioning of the flame retention device, and that particles of smaller sizes, such as, for example, aerosols, are capable of penetrating freely. The risk of clogging by small particles, such as, for example, aerosols, is thus reliably avoided. The pressure loss relevant for the flow behavior of the gas mixture through the recombination apparatus therefore remains particularly low, even after a lengthy period of operation. Moreover, with dimensioning of this kind, in combination with a suitably selected onflow surface, it is possible to configure the recombination apparatus for a particularly favorable flow velocity of the inflowing gas mixture with a view to the hydrogen recombination rate.
The invention proceeds from the knowledge that unintentional ignition of the gas mixture in the vicinity of the recombination apparatus could be triggered by a flame generated during recombination and emerging from the latter. In order to avoid this in a reliable way, the interior of the recombination apparatus, the interior being enclosed by the housing, should be uncoupled in terms of explosion from the space outside the recombination apparatus. A flame retention device is provided for this purpose. In this case, the flame retention device should be disposed in such a way that, in particular, flame propagation opposite to the direction of flow of the gas mixture is reliably prevented.
The flame retention device may be configured as a perforated plate or grid surrounding the housing completely or partially. The flame retention device preferably precedes the catalyst configuration.
In order to avoid unintentional ignition of the gas mixture surrounding the recombination apparatus in a particularly reliable way, the catalyst configuration is advantageously preceded by a sediment trap.
As emerged surprisingly in this regard, even so-called xe2x80x9cstrayxe2x80x9d hot catalyst particles may contribute to ignition of the gas mixture surrounding the recombinator apparatus. When the recombination apparatus is in operation or even shut down, particles may be detached from the catalyst configuration. These detached catalyst particles may have a high temperature due to the heat released during the recombination of the hydrogen and may thus cause ignition when they enter an ignitable gas mixture, for example in the vicinity of the recombination apparatus. In order to avoid ignition of this kind caused by stray catalyst particles discharged from the recombination apparatus, the sediment trap is provided. In this case, the sediment trap is expediently integrated into the flame retention device.
In order, in a particularly reliable way, to prevent the gas mixture which surrounds the recombination apparatus from being ignited by stray hot catalyst particles, the sediment trap can advantageously be cooled by the gas mixture flowing into the housing. This ensures that an incandescent catalyst particle impinging on the sediment trap is cooled immediately and reliably in a particularly simple way.
Expediently, the housing of the recombination apparatus has a housing roof above an outflow orifice for the gas mixture. A housing of well-like construction, which is particularly suitable for the gas mixture to flow through by free convection, can therefore also be used in combination with a spray system disposed overhead, without the possibility of drops acting directly on the catalyst configuration. Catalyst particles being washed out as a result of the direct action of drops on the catalyst configuration is therefore possible at most to an insignificant extent.
The average size of the inflow orifices is preferably at most 2 mm.
In order to reliably prevent ignition of the gas mixture outside of the recombination apparatus, a deflagration volume delimited by the catalyst configuration and by the flame retention device is, in relation to the volume of the recombinator well or well-like housing, preferably smaller than about 20% of the volume of the recombinator well. In this case, the deflagration volume advantageously limits a flame to a flame length of at most 0.3 m, or an average distance between the flame retention device and the catalyst configuration is at most 0.3 m.
In order to avoid coarse particles from being discharged from the recombination apparatus along with the gas mixture which is heated as a result of the recombination reaction, the catalyst configuration is preferably followed by a coarse-particle trap. In this case, the coarse-particle trap may be configured in such a way that, on the one hand, stray catalyst particles detached from the catalyst configuration are effectively prevented from being discharged from the recombination apparatus and, on the other hand, a mixing effect in the gas mixture flowing through occurs in the region of the coarse-particle trap for the purpose of homogenizing the temperature. In this case, the coarse-particle trap is dimensioned in such a way that particles of small size, such as, for example, atmospheric aerosols, can penetrate freely. Filtering of atmospheric aerosols is not necessary in this case, particularly because precipitation of the aerosols is largely avoided as a result of thermophoresis or temperature-induced repulsion at the catalyst configuration. The risk of clogging by the aerosols is thus reliably avoided, so that a pressure loss inhibiting free convection in the recombination apparatus remains low, even when the recombination apparatus operates for a lengthy period of time. The throughput of the gas mixture through the recombination apparatus is therefore not impaired, even in the case of a lengthy period of operation. For this purpose, the coarse-particle trap advantageously has a number of passage orifices with an average gap width of at least 0.1 mm, preferably of at least 0.2 mm, and of at most 1 mm. In this case, the coarse-particle trap may be configured as a single-layer screen plate or else as a multi-layer screen or fiber configuration or as a combination of the two.
In order to limit the surface temperature of the recombination apparatus to a range well below the ignition temperature of the gas mixture surrounding the recombination apparatus, the housing advantageously has an insulating jacket. In this case, an air gap or else a temperature-resistant and radiation-resistant insulating material may be provided in the manner of a double-jacket configuration. Moreover, to reduce heat transmission by radiation, the inner surface of the housing may be metallized. In this case, the inner surface may be ground in such a way as to produce a three-dimensional mirror effect which keeps the relevant transport of heat by radiation low, particularly in a temperature range of more than 500xc2x0 C.
For cooling the gas mixture emerging from the recombination apparatus to a temperature well below the ignition temperature of the gas mixture surrounding the recombination apparatus, the catalyst configuration is expediently followed by an admixing stage for admixing ambient atmosphere to the gas mixture emerging from the catalyst configuration. In this case, the admixing stage is expediently disposed downstream of the coarse-particle trap in the direction of flow. In this case, for admixing, a number of slit orifices is preferably provided in the housing for supplying ambient atmosphere to the gas mixture flowing within the housing.
In order, in a particularly reliable way, to avoid the gas mixture which surrounds the recombination apparatus from being ignited unintentionally, the recombination apparatus is expediently configured in such a way that the gas mixture flowing out of the catalyst configuration, heated as a result of the recombination of hydrogen, has a temperature below the ignition temperature of the gas mixture surrounding the recombination apparatus. For this purpose, the catalyst configuration is advantageously followed by a static mixer that brings about a homogenization or equalization of the temperature of the gas mixture flowing out of the catalyst configuration. In this case, parts of the gas mixture which are locally heated to a particularly high temperature are mixed with parts of the gas mixture which are locally heated to a particularly low temperature, so that high maximum temperatures are avoided.
For particularly effective recombination of the hydrogen carried in the gas mixture, the catalyst configuration advantageously has platinum and/or palladium as a catalytically active material. The catalyst configuration expediently contains a number of approximately plate shaped catalyst bodies, in each case two adjacent catalyst bodies are disposed at an average distance from one another of at least 0.8 cm and of at most 3 cm. In this case, for a high recombination rate along with a low volume, each catalyst body is advantageously coated with catalyst material on both sides, to form reaction surfaces, the gas mixture being capable of being conducted both over the front and over the rear reaction surface of each catalyst body.
The catalyst bodies are expediently held in a joint holding device. The recombination apparatus can therefore be produced at a particularly low outlay in terms of assembly and installation. In this case, the holding device advantageously has good thermal conductivity, so as to ensure in a particularly simple way that the temperature in the region of the catalyst configuration is equalized.
So that the temperature of the gas mixture flowing out of the catalyst configuration is kept in a particularly reliable way below the ignition temperature of the gas mixture surrounding the recombination apparatus, the catalyst configuration is advantageously configured for oxidation of only a fraction of the hydrogen carried in the gas mixture, preferably for oxidation of a fraction of the hydrogen carried in the gas mixture of less than 70%, preferably less than 50%.
A particularly reliable convection flow within the recombination apparatus can be achieved in that the housing advantageously has, in the direction of flow of the gas mixture, a length of at least 0.4 m, preferably of at least 1 m, and of at most 2 m.
The advantages afforded by the invention are, in particular, that, by use of the flame retention device preceding the catalyst configuration, the propagation of a flame, occurring as a result of the heat released during the recombination of hydrogen, into a spatial region outside the recombination apparatus is reliably avoided. In this case, by suitably dimensioning the flame retention device, it is possible to configure the recombination apparatus for a particularly low pressure loss, so that the recombination apparatus can be operated by free convection and therefore by simple measures. Moreover, by virtue of the sediment trap expediently preceding the catalyst configuration and advantageously integrated into the flame retention device, discharge of stray hot catalyst particles that become detached into the spatial region outside the recombination apparatus is reliably avoided. Flameless operation of the recombination apparatus in an ignitable atmosphere, with unintentional ignitions in the spatial region outside the recombination apparatus being avoided, is thus possible.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus for the recombination of hydrogen in a gas mixture, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.